1. Field of the Invention
The present invention relates to a resin sealing structure for flip-chip mounting an IC chip face-down onto a printed circuit board and also to a sealing resin and a method of injecting resin that are used in flip-chip mounting.
2. Description of Related Art
In the past, the method of flip-chip mounting an IC chip to a printed circuit board in a face-down manner is well known. An example of a structure for mounting a semiconductor device by flip-chip mounting is shown in FIG. 4.
Specifically, in the flip-chip mounting used in the past, metal bumps 6 which are formed on aluminum pad electrodes of an IC chip 2 are generally joined to mounting pads on a printed circuit board 1 via a joining material.
Then, to alleviate the stress placed on the joining part which occurs because of the difference in coefficient of thermal expansion between the printed circuit board 1 and the IC chip 2, a sealing resin 3 is injected into a space formed between the printed circuit board surface and a surface of the IC chip 2, after which appropriate thermal treating is done to harden the resin 3 thereby achieving sealing.
The sealing resin 3 suppresses the intrusion of water content or impurity ions from the printed circuit board 1 or the outside, into the IC chip circuit surface and into the aluminum pads, thereby serving to improve reliability in the presence of moisture.
In the prior art, because the coefficient of thermal expansion of the above-noted sealing resin 3 and that of the IC chip 2 were generally different, and because thermal stress would be applied to the semiconductor device, the difference in coefficient of thermal expansion between the sealing resin 3 and the IC chip 2, would cause deformation of a different type. This causes a concentration of stress on the joining part, which often results in peeling at the joining part therebetween or the occurrence of cracks in the resin part, these representing product defects.
For the above-noted reason, in the past, a filler component having an appropriate size was generally added to the sealing resin that serves as a base, the amount of filler component or particle size thereof being adjusted so as to achieve the same or approximately same coefficient of thermal expansion in the sealing resin 3 and IC chip 2.
In the Japanese Unexamined Patent Publication (KOKAI) No. 7-45754, as shown in FIG. 5, discloses of the use of a filler within the sealing resin, this filler having distribution of two types of particle diameters, one larger than the spacing between the printed circuit board 12 and the IC chip 19 and one smaller than this spacing, a resin layer that includes only a filler having a particle diameter that is smaller than the spacing between the printed circuit board and the IC chip being formed in the space between the printed circuit board and the IC chip, so that even when thermal shrinkage of the resin layer because of the heat generated when the IC operates, the distance between the printed circuit board and the IC chip does not become smaller than the included filler particles, thereby preventing damage, for example, to the electrode pattern formed on the surface of the IC chip.
In the prior art, however, in order to solve the problems in a configuration in which entire IC chip 19 is covered with a sealing resin, all that is indicated is a technology for mixing the filler components throughout the entire resin, and this does not disclose, for example, technology related to the solution of the specific problem that exists between the IC chip 2 and the sealing resin 3 in the flip-chip mounting method.
As described above, the sealing resin used in flip-chip mounting, in addition to the purpose of alleviating the stress placed upon the joining part, also has the object of increasing the reliability in the presence of moisture, and a particularly strong, intimate contact is required between the sealing resin and the printed circuit board in order to suppress the absorption of moisture from the boundary between the sealing resin and the printed circuit board.
In order to improve the intimate contact strength between the sealing resin and the printed circuit board, it is generally effective to reduce the amount of filler added to the resin.
This, however, increases the coefficient of thermal expansion of the overall resin, thereby increasing the difference with respect to the coefficient of thermal expansion of the IC chip, the result being that, with respect to thermal stress that occurs because of expansion and contraction, there is a concentration of stress at the joint between the aluminum pad electrodes of the IC chip and the metal bumps, which is a particularly weak joint, this causing cracks and the like in the joint, thereby lowering the reliability with respect to thermal stress, this being at odds with the goal of achieving an improvement in reliability in the presence of moisture.
Additionally, increasing the amount of filler which must be added to make the coefficient of thermal expansion of the sealing resin approach that of the IC chip results in an increase in the viscosity of the sealing resin. The increased viscosity results in the problem of hindering the ease of injection of the sealing resin.
Thus, simply increasing the amount of filler added to the sealing resin results in an increase the viscosity of the resin, this leading to the problem of more difficult injection thereof.
The above-noted problems are encompassed also in the same manner in the Japanese Unexamined Patent Publication (KOKAI) No. 7-45754, under conditions of thermal stress by expansion and contraction.
An object of the present invention is to provide a sealing resin structure, and a sealing resin injection method which, in a flip-chip mounting method for mounting an IC chip in face-down manner to a printed circuit board, solves the problem described above in the prior art.